The following are the specific objectives for this research award application: 1. To detect and characterize differentiation, organ and tissue specific antigens of human lymphoid, myeloid and monocytic cells. 2. To isolate and characterize a human monokine called lymphocyte activating factor (LAF) and to develop human target cell assays for its biological activity. 3. To isolate and characterize the factor(s) present in "conditioned medium" which is required for the initiation and maintenance of long term cultures of human T lymphocytes. 4. To characterize the functional and antigenic nature of the Ia antigen positive human T cells maintained in long term cultures in the presence of conditioned medium. 5. To establish the specificity of selected chimpanzee antisera to purified human HLA-DRw antigens and to characterize the phylogeny of these antigens in the higher apes.